This invention relates to electroless deposition of gold onto ceramic oxide superconductor particles comprising yttrium, rare earth, bismuth, or thallium-alkaline earth metal-copper oxides, and encapsulation of such gold coated ceramic oxide particles between metal sheets or ribbons or in a metal tube, where the gold deposit prevents deleterious reactions at the particle surfaces during and subsequent to fabrication into superconducting tape or wire.
Superconducting materials have been known since 1911, but even modern niobium or vanadium alloys require extremely expensive cooling with liquid helium, which boils at 4.degree. K., in order to operate in the superconducting mode. U.S. Pat. Nos. 4,411,959 and 4,575,927 (Braginski et al.) and 4,050,147 (Winter et al.), teach submicrometer, particulate niobium or vanadium compounds for use in fabricating superconducting wires. Braginski et al. utilize essentially contiguous, submicrometer, superconducting particles, with optional addition of up to 10 volume percent lubricant particles selected from C, MoS.sub.2, Cu, Sn, and Ag. Winter et al. mixes superconducting particles with an equal or greater volume of particles of non-superconducting metals, selected from Cu, Ag, Au, or Al, to provide a major metal component matrix around the superconducting particles. All of the non-superconducting particles are smaller than 500 Angstroms approximate diameter, i.e., 0.05 micrometer, and so the distance between the Nb or V containing particles is short.
Perovskite related ceramic oxides, comprising yttrium-alkaline earth metal-copper oxide, such as orthorhombic, yttrium-barium-copper oxide materials, usually characterized as YBa.sub.2 Cu.sub.3 O.sub.7-x or "1:2:3 ceramic oxides", are well-known "high temperature"superconductor materials. This 1:2:3 ceramic oxide material has been found to provide electrical superconductivity, i.e., essentially no electrical resistance, in the region of 93.degree. K. and below.
The 1:2:3 ceramic oxides and other alkaline earth metal-copper oxide based ceramics can operate in the superconducting mode well above the 77.degree. K. boiling point of relatively inexpensive and plentiful liquid nitrogen, and could find increased use in thin film, high speed switching devices, such as Josephson junctions, high current magnets, energy storage coils, long distance power transmission, and the like. At 93.degree. K., electrons in the 1:2:3 ceramic oxides are thought to travel through the layered crystal lattice as Cooper pairs, rather than individually, with few collisions with the lattice atoms. The Cooper pairs can also tunnel through certain non-superconducting metals disposed between superconducting particles, provided the tunneling distance is short.
These 1:2:3 ceramic oxide superconductors are heat sensitive, losing oxygen from the crystal lattice at temperatures over approximately 700.degree. C., in an orthorhombic-to-tetragonal transition. This can cause problems during wire and ribbon fabrication at temperatures greater than ambient. The 1:2:3 ceramic oxides are very reactive with water, and reactive at moderate temperatures with most metals including Pt and Pd. The 1:2:3 ceramic oxides and rare earth-alkaline earth metal-copper oxide based ceramics are also very brittle and incapable of being formed into wire or windings by conventional methods such as drawing or extrusion.
In another area, non-aqueous electroless deposition of metals from Groups VIII, IB, IVB, VB, VIB, VIIB, and IIIA of the Periodic Chart, onto, for example, porous or non-porous catalytic surfaced powders or granules having diameters up to 0.63 cm., such as Pt coated alumina, is taught in U.S. Pat. No. 3,635,761 (Haag et al.) In this deposition method, a metal pi-complex is reduced. In the complex, the central metal atom is bonded to at least one ligand in the form of an organic group containing at least one carbon-to-carbon multiple bond, and, preferably, at least one singly charged ligand, such as a halide ion, is present. An example of the complex would be 1, 5-cyclo octadiene gold (III) trichloride. Reducing agents are used for decomposing the complex. The metal pi-complex is heated in the presence of the reducing agent, solvent, and catalytic surfaced powder, granule or other substrate, thus depositing free metal. This process, however, involves very complicated metal pi-complex starting materials.
In a method of removing surface oxidation from submicron niobium superconducting particles, such as NbC, Nb.sub.3 Al, NbCN and Nb.sub.3 Sn, U.S. Pat. No. 4,748,737 (Charles et al.), teaches removing niobium oxide from the particles using aqueous alkali metal hydroxide, temporarily protecting the cleaned particles in aqueous hydrazine, and electroless coating of the particles with copper, silver, tin, gold, platinum or the like in a bath of aqueous solution; or by removing oxide and protecting by alkali metal contact, and reduction of a salt of the desired metal in molten alkali metal. As a final step, the coated particles are annealed in hydrogen.
What is needed, is a simple method to prevent loss of oxygen from superconducting ceramic oxide particles comprising alkaline earth metal-copper oxides during heat treatment or wire or ribbon fabrication, and to still allow close contact of the particles in wire or ribbon form, without contacting water-reactive particles with water, which has been found to deteriorate the electrical properties of some alkaline earth metal-copper oxide based ceramic oxides. It is the main object of this invention to provide such a simple method.